These models introduced a new concept of evolutionary stability \(ESS_N\)  adapted from the definition used under infinite populations [1]}. An evolutionarily stable strategy became one on which selection opposes both invasion and fixation by other strategies [2]}, [3]}. These two requirements respectively provide equilibrium and stability conditions [4]} of a given strategy. Selection opposes invasion when no single mutant holds a higher fitness than the residents in a otherwise pure population. Selection opposes fixation when no single mutant is able to fixate in the population with a probability above neutral fixation [3]}. It was shown that of the 16 possible combinations of selection favouring or opposing the invasion and fixation of strategists \(B\)  by strategists \(A\)  and vice-versa, only 8 of those are possible evolutionary scenarios under the frequency-dependent Moran process [6]}.
